From the moment the Voyager probes beamed back the first high-resolution photos of Jupiter’s moon Europa back in 1979, planetary scientists began thinking seriously that this frigid world, a half-billion miles or so from the warming rays of the sun, could be hospitable to life. They already knew Europa was covered in a miles-thick crust of ice, making it one of the most highly reflective objects in the solar system. But close-up images revealed that the ice is riddled with cracks, suggesting that underneath lies an ocean of liquid water — the most basic requirement for life as we know it.

So far, the evidence for that moon-girdling ocean has been compelling, but indirect, making it difficult for scientists to be certain about how friendly to life the water’s chemistry really is. But according to a new paper in Science, that may just have changed in a dramatic way: using the Hubble Space Telescope, astronomers have spotted what appear to be plumes of water spurting more than 100 miles above Europa’s surface like huge, chilly geysers.

“It could be a false positive,” says a cautious Kevin Hand, a planetary scientist with NASA’s Jet Propulsion Laboratory, who wasn’t part of the team, “but it looks robust.” “If this paper is correct,” he says, “then Europa is geologically active today. The next question is, What’s in Europa’s ocean and could it — or does it — harbor life?”

The answer to that crucial question won’t be coming anytime soon, but the observation of water in Europa’s oceans is a big step in the right direction. In fact, says planetary scientist Lorenz Roth of the Southwest Research Institute in Boulder, Colo., the paper’s lead author, “the idea has been around for some time that plumes of water could exist on Europa.” That’s because the moon is subjected to constantly changing tidal forces from Jupiter and from the planet’s other large moons.

Internal friction from this tidal squeezing keeps Europa’s core warm, which keeps the ocean from freezing solid and could in theory send some of that water out into space. That’s exactly what seems to be happening on Saturn’s moons Enceladus, and perhaps Titan as well. It’s also happening on Jupiter’s moon Io — except that in Io’s case, the eruptions, first spotted by Voyager in 1979, are more rocky than watery.

Neither the Voyagers nor the Galileo spacecraft saw any such activity on Europa, but astronomers realized that such eruptions could be highly intermittent, and therefore easy to miss. In 1999, they took a look with Hubble and saw nothing, but about a year ago, Roth and his colleagues decided to try again. This time, they got lucky. Using an instrument called the Space Telescope Imaging Spectrograph, they detected the faint glow of ultraviolet light above Europa’s southern hemisphere.

Further analysis suggested the glow came from oxygen and hydrogen atoms being bombarded by particles being whipped around by Jupiter’s intense magnetic fields. The glow persisted for at least seven hours, at the point where Europa was furthest from Jupiter in its 3.5-day orbit and the tidal squeezing greatest. When Europa was closer to Jupiter, the glow vanished.

Surprisingly, says Hand, “this part of Europa is not where I would have predicted we’d see active plumes. If anything, models indicate that the ice should be thicker toward the poles, and thicker ice is harder to crack.” The conventional wisdom says the ice layer is at least a few miles thick on average, and maybe tens of miles thick. But this observation could be telling us, says Hand, that it’s significantly thinner. And if that’s the case, drilling down to look around and take samples in some future Europa mission would be easier than anyone currently believes.

It’s also possible that the plumes of water could be coming not from Europa’s ocean, but from a lake or lakes nestled within the ice shell itself. “There’s no way to tell at this point,” says Hand. If it’s the latter, then the plumes may not be all that useful in revealing what sorts of salts or other minerals are dissolved in Europa’s ocean, and thus whether conditions are pleasant or poisonous.

As it happens, Hand is already working out the details of a proposed mission called the Europa Clipper, which would do no fewer than 32 flybys, and which could presumably target the plumes. “We need to get a spacecraft back out to this fascinating world to better explore and understand its potentially habitable ocean,” he says. And while the mission hasn’t been approved yet by NASA, this new discovery can hardly hurt its chances.